I am assuming you have the algae off and you're referring to a brown tint on the hull. I have used Sno Bowl toliet bowl cleaner which was recommended by the owner of my marina and it works great and is less expensive than products from marine shops. ON/OFF, Zing and Muriatic Acid will take Algae off and whiten hull.

Whatever you use wear rubber gloves in well ventilated area. Also, clean on a cleaning rack at Marina if possible otherwise the cleaniers will damage your trailer's finish.